1. Technical Field
The present invention relates to an electrophotographic photoreceptor, a process cartridge, and an image forming apparatus.
2. Related Art
Generally, an electrophotographic image forming apparatus has the following configurations and processes.
That is, a surface of an electrophotographic photoreceptor is charged to desired polarity and potential by a charging unit, and the charge is selectively erased from a charged surface of the electrophotographic photoreceptor by image-wise exposure, thereby forming an electrostatic latent image. The latent image is then developed into a toner image by adhering a toner to the electrostatic latent image by a developing unit, the toner image is transferred onto a transfer medium by a transfer unit, and then the transfer medium is discharged as an image formed material.
Electrophotographic photoreceptors are currently been increasingly used in the field of copying machines, laser beam printers, and the like due to advantages of obtaining high printing quality with a high speed.
As electrophotographic photoreceptors used in image forming apparatus, electrophotographic photoreceptors used in the related art, using inorganic photoconductive materials such as selenium, selenium-tellurium alloy, selenium-arsenic alloy, and cadmium sulfide (inorganic photoreceptors) have been known, but recently, organic photoreceptors using organic photoconductive materials having superior advantages in views of low cost, manufacturability, and disposability (organic photoreceptors) are mainly used.
As a charging system that charges the surface of an electrophotographic photoreceptor, a corona charging system utilizing a corona discharger has been used in the related art. However, a contact charging system having advantages such as low ozone production and low electricity consumption has recently been put into practical use and is widely used. In this contact charging system, the surface of a photoreceptor is charged by bringing an electroconductive member as a charging member into contact with, or in close proximity to, the surface of the photoreceptor, and applying a voltage to the charging member. As a system for applying a voltage to the charging member, there are a direct current system in which only a direct current voltage is applied, and an alternating current superimposition system in which a direct current voltage superimposed by an alternating current voltage is applied. The contact charging system has advantages of downsizing the apparatus as well as suppressing generation of gases such as ozone.
As a transfer system that transfers a toner image onto a transfer medium, a system in which a toner image is transferred directly to paper has been mainly used, but a system of transferring a toner image to paper via an intermediate transfer member is frequently used in recent years due to a wider variety of types of paper for transfer.
Furthermore, in order to obtain a high-resolution image, investigations are recently conducted to obtain a more precise image by providing a toner with a small diameter. It is preferable to make a toner with a small diameter for the purpose of obtaining a high-precision image, but as the diameter of the toner decreases, the transfer to a transfer medium from a photoreceptor becomes difficult. Hence, for example, a method is taken, in which a pressing pressure between the photoreceptor and the transfer medium is increased or some differences in peripheral speeds between the photoreceptor and the transfer medium are given to make an easier mechanic transfer. As the transfer medium, a flexible belt is generally used, but it is apparent that there is still a desire for a long lifetime of the belt, such as low dependency of a resistance value on environment, a less tensile stretch, high fracture strength such as cracks, and high abrasion resistance. In this regard, a high-strength polyimide is used in many cases. The polyimide has a significantly high strength, and thus, a unit for improving the transfer efficiency of the toner is accompanied by a significantly high stress on a photoreceptor facing the unit. In addition, it is often difficult to remove the toner remaining on the photoreceptor, so-called cleaning, and thus, for example, a method for increasing the pressing pressure of a cleaning blade onto the photoreceptor is taken. This causes mechanic friction on the surface of the photoreceptor. This mechanical stress brings about an increase in the cutting of the surface layer of the photoreceptor, which is a main cause of making the lifetime of the photoreceptor shorter, and a photoreceptor excellent in the mechanic strength is critical for a long lifetime and high reliability.
In order to attain a longer lifetime and higher reliability of the electrophotographic photoreceptor, it has been proposed to provide a protective layer on the surface of an electrophotographic photoreceptor to improve the strength.
The materials for forming a protective layer have been proposed.